Fuel fractioning unit for inert gas generating system

ABSTRACT

An inert gas generating system includes a source of a liquid hydrocarbon fuel, and a fractioning unit configured to receive a portion of the liquid hydrocarbon fuel from the source. The fractioning unit includes a perm-selective membrane configured to separate the portion of the liquid hydrocarbon fuel into substantially sulfur-free vapors and a sulfur-containing remainder. The system further includes a catalytic oxidation unit configured to receive and react the substantially sulfur-free vapors to produce an inert gas.

BACKGROUND

Fuel tanks can contain potentially combustible combinations of oxygen,fuel vapors, and ignition sources. In order to prevent combustion inaircraft fuel tanks, aviation regulations require actively managing theullage of fuel tanks, such that the oxygen partial pressure in theullage is less than 12%. Relatedly, fire suppression systems, such asthose deployed in aircraft cargo holds, use halogenated chemicals toprevent combustion and/or fire. Halogenated fire suppression agents canbe safe for human exposure; however, they are known to be detrimental tothe Earth's atmospheric ozone layer. Inert air can be used for fireprevention and suppression.

Currently, many On-Board Inert Gas Generation Systems (OBIGGS) use bleedair and pressurized hollow fiber membranes to produce inert gas for fueltank ullages. In hollow fiber membranes, the diffusivity of nitrogen isless than the diffusivity of oxygen and water vapor. Hollow fibermembrane systems require pressurized air to drive the separation ofnitrogen from oxygen and water vapor in an air stream. However, thepressure of bleed air extracted from an aircraft engine compressorvaries throughout a mission, which affects inert gas production quantityand quality as defined by oxygen partial pressure. Furthermore, aircraftdesign is trending toward lower pressure bleed systems and increasinglyelectric power distribution architectures. Accordingly, the use of highpressure, hollow fiber membrane inerting systems can be problematic forthese systems.

Other approaches utilize catalytic reactors to produce inert gas fromullage space fuel vapors, or from liquid fuel. The ullage space,however, may not always contain a sufficient amount of fuel vapors toprovide for reaction. Thus, a system capable of maintaining a safeoxygen partial pressure in the ullage is necessary in order to complywith regulations requiring ullage passivation throughout the mission.

SUMMARY

An inert gas generating system includes a source of a liquid hydrocarbonfuel, and a fractioning unit configured to receive a portion of theliquid hydrocarbon fuel from the source. The fractioning unit includes aperm-selective membrane configured to separate the portion of the liquidhydrocarbon fuel into substantially sulfur-free vapors and asulfur-containing remainder. The system further includes a catalyticoxidation unit configured to receive and react the substantiallysulfur-free vapors to produce an inert gas.

A method for generating inert gas includes: providing a liquidhydrocarbon fuel to a fractioning unit, the fractioning unit having aperm-selective membrane; creating a partial pressure gradient such thata partial pressure of substantially sulfur-free vapors of the liquidhydrocarbon fuel is lower on a permeate side of the perm-selectivemembrane; providing an amount of the substantially sulfur-free vaporsdrawn through the perm-selective membrane to a catalytic oxidation unit;and reacting the amount of the substantially sulfur-free vapors toproduce the inert gas.

An alternative embodiment of an inert gas generating system includes asource of liquid hydrocarbon fuel, and a fractioning unit configured toreceive a portion of the liquid hydrocarbon fuel and to separate theportion of the liquid hydrocarbon fuel into substantially sulfur-freevapors and a sulfur-containing remainder. A catalytic oxidation unit isconfigured to receive and react the substantially sulfur-free vapors toproduce the inert gas. The fractioning unit is further configured tooutput the sulfur-containing remainder into a return line. The returnline is configured to provide the sulfur-containing remainder to a mainfuel line, the main fuel line including an engine-bound fuel flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an inert gas generating system.

FIG. 2 is a schematic view of a fractioning unit of the inert gasgenerating system.

FIG. 3 is a schematic view of an alternative embodiment of thefractioning unit.

FIG. 4 is a schematic view of another alternative embodiment of thefractioning unit.

DETAILED DESCRIPTION

The present disclosure relates to inert gas generating systems andmethods of use thereof for generating an inert gas through catalyticoxidation. A fractioning unit produces substantially sulfur-free fuelvapors to undergo catalytic oxidation. The fractioning unit is capableof operating without significant pressurized air input.

FIG. 1 is a schematic view of inert gas generating system 10. System 10includes fuel tank 12, liquid fuel 14, fuel pump 16, fractioning unit18, catalytic oxidation unit 20, cooling unit 22, dryer 24, passage 26,ullage space 28, vent 30 and, optionally, cargo hold 32. Fuel tank 12provides liquid fuel 14 to fractioning unit 18 via fuel pump 16. System10 can include one, or a plurality of fractioning units 18. Desired fuelfractions produced at fractioning unit 18 are then provided to catalyticoxidation unit 20. These hydrocarbon-rich fuel fractions are reacted incatalytic oxidation unit 20 to produce an inert gas containing primarilynitrogen, carbon dioxide and water vapor. The catalyst used in thereaction can be, for example, a noble metal, or other suitable catalyst.Cooling unit 22 and dryer 24 condition the inert gas by cooling it andremoving the water vapor before its introduction to ullage space 28 viapassage 26. System 10 is configured to provide the unused fuel fractionsto the engines, or other locations.

Liquid fuel 14 can be a kerosene-based jet fuel, such as Jet-A, Jet-A1,or Jet-B fuel. For military applications, liquid fuel 14 can also be ajet propulsion “JP” class fuel, such as JP-5 or JP-8. Other types offuel such as diesel, gasoline, and mixtures of fuels are alsocontemplated herein. In the embodiment shown, fuel tank 12 serves as theliquid fuel source, but in other embodiments, other fuel storage vesselsor liquid fuel sources can be used. Similarly, fuel pump 16 is shownexternal to fuel tank 12 in FIG. 1, however, fuel pump 16 can also belocated within fuel tank 12.

Ullage space 28—the vapor space above liquid fuel 14 within fuel tank12—can contain potentially combustible fuel vapors. System 10 operatesto reduce the risk of combustion within ullage space 28 by providinginert gas to maintain the oxygen concentration within ullage space 28 ator below 12% oxygen by volume to meet commercial aviation requirements,and below 9% for military applications.

FIG. 2 is a schematic view of fractioning unit 118, which is a membranefractioning unit. Fractioning unit 118 includes membrane 134, which is aperm-selective membrane configured to selectively permeate hydrocarboncompounds over, for example, oxygen and nitrogen. Membrane 134 can be areverse selective membrane formed from a cross-linked silicone rubber,such as polydimethylsiloxane (PDMS). Other suitable materials caninclude polyoctylmethyl siloxane, polyethers (e.g., copolymers ofpoly(ethylene oxide) (PEO) and poly(butylene therephthalate) (PBT)),poly(4-methyl-2-pentyne), and poly-trimethyl-silyl-propyne (PTMSP).Membrane 134 can further be arranged as a plurality of hollow fibers, orcan be in planar (sheet) form and have a “spiral-wound” or“plate-and-frame” configuration. Other suitable materials andconfigurations are contemplated herein.

In operation, liquid fuel 114 is provided to fractioning unit 118 byfuel pump 116. Liquid fuel 114 contains numerous compounds andadditives, including the hydrocarbon compounds (paraffins,cycloparaffins or naphthenes, aromatics, olefins, etc.) provided forreaction within catalytic oxidation unit 20 (not shown in FIG. 2).Liquid fuel 114 can also contain sulfur compounds (sulfides, thiols,thiophenes, etc.). These compounds can poison the reaction catalyst (notshown), such that they bind to active sites on the catalyst and reducethe amount of active sites available for promoting the reaction. Amajority of the sulfur within liquid fuel 114 is contained in specieshaving large molecular diameters. Accordingly, membrane 134 isconfigured to disfavor by preferentially preventing passage of manysulfur-containing compounds therethrough by excluding larger, cyclicmolecules, such as aromatic sulfur compounds.

The portion of liquid fuel 114 contained within fractioning unit 118 isdenoted liquid fuel 114 _(F) to distinguish it from liquid fuel 114within fuel tank 112 (not shown in FIG. 2). Depending on the stage ofthe fractioning process, the composition of liquid fuel 114 _(F) canvary from that of liquid fuel 114. For example, as the fractioningprocess progresses, liquid fuel 114 _(F) will become enriched withsulfur compounds, as is explained in more detail below.

In order to drive permeation of hydrocarbon vapors 136 across membrane134, the partial pressure of hydrocarbon vapors 136 should be lower onpermeate side 138 of membrane 134. In the embodiment of FIG. 2, membranefractionation is driven by a sweep gas. Air source 140 acts to generatea pressure differential by providing air stream 142—the sweep gas—tofractioning unit 118. Air source 140 can be any type of air source, forexample, ambient air, ram air, fan air, engine bleed air, or cabin airwith appropriate thermal regulation. Other suitable air sources arecontemplated herein.

Air stream 142 flows across permeate side 138 of membrane 134. Airstream 142 does not contain hydrocarbons, so the initial partialpressure of hydrocarbon vapors 136 on permeate side 138 is zero. Thepartial pressure differential across membrane 134 initiates thetransport of hydrocarbon vapors 136 across membrane 134. Hydrocarbonvapors 136 are substantially sulfur-free, as a majority of the sulfurremains in the heavier, unevaporated fractions of liquid fuel 114 _(F).Hydrocarbon vapors 136 are subsequently provided to catalytic oxidationunit 120.

FIG. 3 is a schematic view of fractioning unit 218, which is anotherembodiment of a membrane fractioning unit. Fractioning unit 218 operatesin the same manner as fractioning unit 118, except that the transport ofhydrocarbon vapors 236 across membrane 234 is driven by vacuum source244. Vacuum source 244 is in fluid communication with permeate side 238of membrane 234, and is configured to generate a partial pressuredifferential, such that the partial pressure of hydrocarbon vapors 236is lower on permeate side 238 of membrane 234. Vacuum source 244 can be,for example, an ejector, a diaphragm pump, other suitable pump, orcombinations thereof.

In embodiments having membrane fractioning units (118, 218), system 10can optionally include a heat source (not shown) to heat the liquid fuel(114, 214) upstream of the membrane (134, 234). The heat source can be,for example, a heating element or a recuperating heat exchanger.

In general, the production of hydrocarbon vapors from a given volume offuel is a function of the surface area of the fuel available forevaporation, fuel temperature, and the partial pressure above the fuel.Temperature and partial pressure can vary throughout a mission, due tochanges in air speed and altitude. In the embodiments of FIGS. 2 and 3,there are several ways to further regulate the amount of hydrocarbonvapors (136, 236) produced. The first is to vary the amount of liquidfuel (114, 214) provided to the fractioning unit. This can beaccomplished, for example, by placing an actuated valve downstream ofthe fuel pump (116, 216). In the embodiment of FIG. 2, the temperatureand flow rate of the sweep gas (air stream 142) can also be varied tocontrol vapor production. For example, when air stream 142 has a highertemperature and/or a greater flow rate from air source 140, it can helpgenerate and carry more hydrocarbon vapors 136, taking care to balancethe need for fuel vapor with the stoichiometric need of oxygen forcombustion.

The membrane fractioning units described above have several advantagesover prior art systems. First, the configuration of the membraneprovides a robust surface area on which evaporation of the liquid fuelcan occur, thus allowing for a large volume of hydrocarbon vapors to begenerated. The membranes are also configured to selectively permeatespecies having smaller molecular diameters, which facilitates theproduction of substantially sulfur-free vapor fractions. In other typesof fractioning systems, the presence of sulfur within the vaporfractions is harder to control.

It is further possible for inert gas generating systems using membranefractioning to be incorporated into an aircraft fuel system, such that aportion of the fuel flow to the engines can run through each fractioningunit on board. This requires that the total flow rate through thefractioning unit(s) can exceed, for example, 2,500 kg/hr, depending onengine fuel requirements during a mission. Systems using thermalfractioning, for example, would require an immense thermal input totreat such a fuel flow.

FIG. 4 is a schematic view of fractioning unit 318. Fractioning unit 318is configured to receive liquid fuel 314 from fuel tank 312. Fuel tank312 includes a partitioned collector cell 346, in which fuel pump 316 islocated. Fuel pump 316 can be, for example, a feed pump, transfer pump,or any other suitable low-pressure pump. Fuel tank 312 also includesullage space 328, vent 330, and scavenge pump 348.

Fractioning unit 318 is in fluid communication with fuel tank 312 viamain fuel line 350 and branch line 352. Main fuel line 350 is configuredto provide a fuel flow from fuel tank 312 to one or more engines. Branchline 352 is configured to provide a portion of the fuel flow tofractioning unit 318. Branch line 352 can include valve 354 to controlthe fuel flow into fractioning unit 318. Valve 354 can be a needlevalve, or some other type of suitable, actuated valve. Valve 354 can beconfigured to vary the fuel flow into fractioning unit 318 according toa schedule based on mission parameters (ground, cruise, descent, etc.).Other suitable control methods (passive or active) are contemplatedherein.

Fractioning unit 318 can be configured as a vacuum fractioning unit. Insuch an embodiment, vacuum source 344 is in communication withfractioning unit 318. Vacuum source 344 is configured to reduce thepartial pressure above liquid fuel 314 _(F) to produce hydrocarbonvapors 336. Vacuum source 344 further operates to suck hydrocarbonvapors 336 out of the fractioning unit 318 to provide to catalyticoxidation unit 320 (not shown in FIG. 4). In some embodiments, vacuumsource 344 can be a vacuum pump, such as a diaphragm pump, or othersuitable pump. In other embodiments, vacuum source 344 can be an ejectorthat uses hot bleed air as a motive fluid. In other embodiments, vacuumsource 344 can be a series combination of vacuum pumps or ejectors, orcombinations thereof. Other suitable vacuum sources are contemplatedherein. In some embodiments, fractioning unit 318 can include a demister(not shown) upstream of vacuum source 344 to capture droplets of liquidfuel 314 _(F) and prevent them from entering catalytic oxidation unit320.

Fractioning unit 318 can also be in communication with heat source 358,which is used to heat and thereby improve the transport properties ofliquid fuel 314 _(F) within fractioning unit 318. In some embodiments,heat source 358 is used in conjunction with vacuum source 344 tofacilitate the production of hydrocarbon vapors 336. Fractioning unit318 can also be configured as a thermal fractioning unit, and relysolely on heat source 358 to produce hydrocarbon vapors 336. In theembodiment shown, heat source 358 is a heating element in thermalcommunication with fractioning unit 318. In other embodiments, heatsource 358 can be a heat exchanger with a heat source such as bleed air,hot engine oil, hot fuel, or hot inert gas generated by the catalyticoxidation process. The heat sink for the heat exchanger can be liquidfuel 314 _(F) in fractioning unit 318 or liquid fuel 314 bound forfractioning unit 318. This invention additionally envisions any othersuitable heat source.

Heat source 358 can be in communication with fractioning unit 318 in themanner shown in FIG. 4, to heat liquid fuel 314 _(F) within fractioningunit 318. It can also be located, for example, along branch line 352 toheat liquid fuel 314 as it enters fractioning unit 318. Other locationssuitable for heating liquid fuel 314/314 _(F) or other components offractioning unit 318 are contemplated herein.

Whether configured as a vacuum or thermal fractioning unit, fractioningunit 318 produces substantially sulfur-free hydrocarbon vapors 336,because both methods generally separate vapor components by boilingpoint. Sulfur-containing compounds generally have higher boiling pointsthan many of the hydrocarbon compounds within liquid fuel 314 (hexane,benzene, cyclohexane, etc.), so sulfur-containing compounds tend not toevaporate with the hydrocarbon compounds. In other embodiments,fractioning unit 318 can include a membrane 334 (not shown) thatoperates to produce hydrocarbon vapors 336 in the same manner asmembranes 134 and 234.

Fractioning unit 318 is in communication with return line 356. Returnline 356 is configured to receive the unevaporated, sulfur-containingremainder 360 of liquid fuel 314 _(F) from fractioning unit 318, andprovide remainder 360 to main fuel line 350. Return pump 362 isdownstream of fractioning unit 318 along return line 356. Return pump362 is configured to provide remainder 360 to main fuel line 350 at asuitable pressure. Return pump 362 can be an ejector, or other suitablepump. Remainder 360 can be joined with engine-bound liquid fuel 314 fromfuel tank 312 at main line 350.

Fractioning unit 318 has several advantages over prior art systems Likemembrane fractioning units 118 and 218, fractioning unit 318 isconfigured to produce substantially sulfur-free hydrocarbon vapors,which helps mitigate catalyst poisoning. Fractioning unit 318 alsoprovides a method for releasing the sulfur-containing remainder, whichhelps prevent a buildup of sulfur-containing residual fractions withinfractioning unit 318. Providing the remainder for engine consumption isalso beneficial because it ensures that the unused remainder is notwasted. Further, the remainder can be quite hot after leavingfractioning unit 318, so returning it to, for example, fuel tank 312, isundesirable, as it heats the bulk fuel. In addition, the return ofsulfur-containing fuel to a fuel tank undesirably concentrates sulfur inthe tank, which may lead to higher rates of corrosion.

The various embodiments of system 10 can be used for other aircraftapplications, such as cargo hold fire suppression. Current systemsemploy Halon 1301, a capable fire suppressant also known to bedetrimental to the environment. Inert gas produced by system 10 cantherefore be used to reduce or eliminate the amount of Halon 1301 usedin these systems. For example, in addition to/instead of being sent toullage space 28, inert gas can be routed to cargo hold 32 (shown in FIG.1).

System 10 can be used to provide inert gas for fuel sparging. That is,inert gas can be bubbled through the liquid fuel to reduce the amount ofdissolved oxygen and other impurities within the fuel. Sparging has thedual benefit of stabilizing the fuel for use as a heat sink andpassivating the ullage space.

System 10 can also be used in non-aviation applications. For example,system 10 can be used in maritime vessels for purging and inertingstorage tanks for crude oil, natural gas, product hydrocarbons, andvehicle fuel. System 10 can also be used in the automotive or otherindustries having a need for inert gas.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

An inert gas generating system includes a source of a liquid hydrocarbonfuel, and a fractioning unit configured to receive a portion of theliquid hydrocarbon fuel from the source. The fractioning unit includes aperm-selective membrane configured to separate the portion of the liquidhydrocarbon fuel into substantially sulfur-free vapors and asulfur-containing remainder. The system further includes a catalyticoxidation unit configured to receive and react the substantiallysulfur-free vapors to produce an inert gas.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A pressure differential generator is in fluid communication with apermeate side of the perm-selective membrane, and is configured tocreate a partial pressure gradient across the perm-selective membrane.

The partial pressure generator is an air source configured to provide anair stream.

The air stream is selected from the group consisting of ambient air, fanair, engine bleed air, cabin air, and combinations thereof.

The pressure differential generator is a vacuum source.

A fuel pump is configured to provide the liquid hydrocarbon fuel to thefractioning unit.

The perm-selective membrane is a reverse selective membrane and includesa plurality of hollow fibers.

The perm-selective membrane is a reverse selective membrane in planarform having a spiral-wound or plate-and-frame configuration.

The perm-selective membrane is a silicone-based material.

A passageway is configured to provide the inert gas to a defined space.

The defined space is a fuel tank or a cargo hold.

The fractioning unit is configured to provide the sulfur-containingremainder to a main fuel line, and the main fuel line comprises anengine-bound fuel flow.

A method for generating inert gas includes: providing a liquidhydrocarbon fuel to a fractioning unit, the fractioning unit having aperm-selective membrane; creating a partial pressure gradient such thata partial pressure of substantially sulfur-free vapors of the liquidhydrocarbon fuel is lower on a permeate side of the perm-selectivemembrane; providing an amount of the substantially sulfur-free vaporsdrawn through the perm-selective membrane to a catalytic oxidation unit;and reacting the amount of the substantially sulfur-free vapors toproduce the inert gas.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

Generating the partial pressure gradient includes flowing an air streamover the permeate side of the perm-selective membrane.

Generating the partial pressure gradient comprises operating a vacuumsource in fluid communication with the permeate side of theperm-selective membrane.

An alternative embodiment of an inert gas generating system includes asource of liquid hydrocarbon fuel, and a fractioning unit configured toreceive a portion of the liquid hydrocarbon fuel and to separate theportion of the liquid hydrocarbon fuel into substantially sulfur-freevapors and a sulfur-containing remainder. A catalytic oxidation unit isconfigured to receive and react the substantially sulfur-free vapors toproduce the inert gas. The fractioning unit is further configured tooutput the sulfur-containing remainder into a return line. The returnline is configured to provide the sulfur-containing remainder to a mainfuel line, the main fuel line including an engine-bound fuel flow.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A heat source is configured to elevate a temperature of the portion ofthe liquid hydrocarbon fuel.

A vacuum source is in communication with the fractioning unit, and thevacuum source is configured to reduce a partial pressure of the portionof the liquid hydrocarbon fuel.

The vacuum source is one of a vacuum pump or an ejector.

The fractioning unit further includes a perm-selective membrane, and themembrane is configured to selectively permeate the substantiallysulfur-free vapors.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. An inert gas generating system comprising:a source of a liquid hydrocarbon fuel; a fractioning unit configured toreceive a portion of the liquid hydrocarbon fuel from the source, thefractioning unit comprising a perm-selective membrane configured toseparate the portion of the liquid hydrocarbon fuel into substantiallysulfur-free vapors and a sulfur-containing remainder; a pressuredifferential generator in fluid communication with a permeate side ofthe perm-selective membrane, wherein the pressure differential generatoris configured to create a partial pressure gradient across theperm-selective membrane; and a catalytic oxidation unit configured toreceive and react the substantially sulfur-free vapors to produce aninert gas.
 2. The system of claim 1, wherein the pressure differentialgenerator is an air source configured to provide an air stream.
 3. Thesystem of claim 2, wherein the air stream is selected from the groupconsisting of ambient air, ram air, fan air, engine bleed air, cabinair, and combinations thereof.
 4. The system of claim 1, wherein thepressure differential generator is a vacuum source.
 5. The system ofclaim 1 further comprising: a fuel pump configured to provide theportion of the liquid hydrocarbon fuel to the fractioning unit.
 6. Thesystem of claim 1, wherein the perm-selective membrane is a reverseselective membrane comprising a plurality of hollow fibers.
 7. Thesystem of claim 1, wherein the perm-selective membrane is a reverseselective membrane in planar form having a spiral-wound orplate-and-frame configuration.
 8. The system of claim 1, wherein theperm-selective membrane comprises a silicone-based material.
 9. Thesystem of claim 1 further comprising: an inert gas passageway configuredto provide the inert gas to a defined space.
 10. The system of claim 9,wherein the defined space is one of a fuel tank or a cargo hold.
 11. Thesystem of claim 1, wherein the fractioning unit is configured to providethe sulfur-containing remainder to a main fuel line, and wherein themain fuel line comprises an engine-bound fuel flow.
 12. A method forgenerating inert gas, the method comprising: providing a portion ofliquid hydrocarbon fuel from a fuel source to a fractioning unit, thefractioning unit comprising a perm-selective membrane configured toseparate the portion of the liquid hydrocarbon fuel into substantiallysulfur-free vapors and a sulfur-containing remainder; creating a partialpressure gradient using a pressure differential generator in fluidcommunication with a permeate side of the perm-selective membrane suchthat a partial pressure of the substantially sulfur-free vapors of theliquid hydrocarbon fuel is lower on the permeate side of theperm-selective membrane; providing an amount of the substantiallysulfur-free vapors drawn through the perm-selective membrane to acatalytic oxidation unit; and reacting the amount of the substantiallysulfur-free vapors to produce the inert gas.
 13. The method of claim 12,wherein generating the partial pressure gradient comprises flowing anair stream over the permeate side of the perm-selective membrane. 14.The method of claim 12, wherein generating the partial pressure gradientcomprises operating a vacuum source in fluid communication with thepermeate side of the perm-selective membrane.